1. Field of the Invention
The present invention relates to a method of controlling automatic focusing in a video camera. More particularly, the present invention pertains to a method of controlling automatic focusing, which is suitable for photographing night views.
2. Description of the Related Art
The recently developed video camera for household use has rapidly improved its picture quality. With this improvement, the performance has also improved in automatic controlling such as automatic focusing (hereinafter refereed to as AF).
Most AF is performed using the contrast detecting method which is called uphill AF, and the performance depends on the clearness of an object. A case in point, when the object such as a night view, of which the luminance (illuminance) is low on the whole screen, is photographed, the performance deteriorates significantly. Depending on the situation, an error takes place sometimes, causing the object to be out of focus.
Conventionally, if the object such as the night view has an extremely low luminance, an automatic gain control (AGC) is performed so as to amplify a video signal to make a contrast on an image, or a special mode such as a twilight mode is provided to lower the shutter speed.
The AGC, however, has such a disadvantage in that the noise increases and signal-to-noise ratio lowers. Thereby, a video signal effective for AF control cannot be obtained, and the focusing cannot be accurately performed in many cases. On the other hand, in the case of the photographing in the twilight mode, the image looks like as if the frames were fed one by one. For this reason, even if AF becomes possible, the video image does not look natural.
Manual focusing is performed in some methods, but it has a disadvantage in that it depends on the user's technical skill, and it negates the full automation which is currently the mainstream.